1. Field of the Invention
The present invention relates to ferroelectric thin-film elements formed on Si substrates using thin-film deposition techniques, and sensors using the same, and also relates to methods for making the same.
2. Description of the Related Art
Recently, the demand for elements using ferroelectrics are rapidly increasing. Acceleration sensors using the piezoelectricity of ferroelectric thin-film elements and pyroelectric infrared sensors using the pyroelectricity of ferroelectric thin-film elements are mainly used. However, smaller sensors with higher reliability in comparison with the conventional sensors have been desired.
In order to reduce the sizes of sensors, the elements contained therein must be miniaturized. However, most of the existing ferroelectric thin-film elements (piezoelectric elements and pyroelectric elements) are formed by sintering, molding and machining ferroelectric ceramics, and although attempts have been made to reduce size and increase sensitivity, it is difficult to satisfy the demands for further miniaturization using the conventional techniques.
Therefore, development of a monolithic element combined with a semiconductor substrate is desired in which, using a new method, a sensing part, which is composed of a thin-film laminate including a ferroelectric thin film and metallic thin films for electrodes, is directly formed on the semiconductor substrate using a thin-film deposition technique, and the sensing part is machined by a fine patterning technique so as to have a diaphragm structure, a hollow structure or a cantilever structure. By forming the sensing portion with thin films as thus described, the size can be reduced and it is also possible to form the sensing part and a circuit part collectively on the same semiconductor substrate, and thus the size of the ferroelectric thin-film element can be further reduced. An attempt has also been made to improve the performance of an element by forming a Pb(Zr, Ti)O3 (hereinafter referred to as xe2x80x9cPZTxe2x80x9d) thin film or a (Pb, La)(Zr, Ti)O3 (hereinafter referred to as xe2x80x9cPLZTxe2x80x9d) thin film which has a particularly large remanence polarization and superior piezoelectricity and pyroelectricity on a substrate by oriented growth or epitaxial growth so as to obtain higher polarization.
However, the development of a monolithic ferroelectric thin-film element combined with a semiconductor substrate as described above gives rise to the problems described below. That is, there was a technical difficulty in forming a ferroelectric thin film having satisfactory orientation on a substrate using a thin-film deposition technique. A fine patterning technique such as etching was not established.
Although a ferroelectric thin-film element is disclosed in which a sensing part comprising a ferroelectric thin film and metallic thin films is formed on a single crystalline MgO substrate, the MgO substrate is expensive and is difficult to machine.
Accordingly, the objects of the present invention are to provide a small ferroelectric thin-film element which has high performance and which is inexpensive and easy to machine by using a technique for forming a ferroelectric thin film having superior orientation on a semiconductor substrate and by using various etching techniques on the substrate, metallic thin films and the ferroelectric thin film, and to provide a sensor using the same.
As a result of thorough research, the present inventors have found that by forming a buffer layer composed of MgO, Ti1-xAlxN (where the subscript x is 0 to about 0.4), YSZ, or the like, it is possible to orientationally grow or epitaxially grow a metallic thin film and a ferroelectric thin film in that order on an Si substrate as a semiconductor substrate with the buffer layer therebetween. By using various etching techniques, the Si substrate and the buffer layer are partially removed to form a diaphragm structure, a hollow structure or a cantilever structure, and thus the present invention has been achieved.
In one aspect of the present invention, a ferroelectric thin-film element includes an Si substrate and a thin-film laminate formed on the Si substrate. The thin-film laminate includes a buffer layer epitaxially grown on the Si substrate, a lower electrode composed of a metallic thin film epitaxially grown on the buffer layer, a ferroelectric thin film orientationally grown or epitaxially grown on the lower electrode, and an upper electrode formed on the ferroelectric thin film. A portion of the thin-film laminate is disposed so as to be supported by the air.
A portion of the Si substrate located under the thin-film laminate may be etched to provide a recess or a through-hole in the Si substrate so that the thin-film laminate is disposed so as to be supported by air. Alternatively, a portion of the buffer layer may be etched so that the thin-film laminate is disposed so as to be supported by air. At least a portion of the buffer layer located on the recess or the through-hole of the Si substrate may be further etched so that the thin-film laminate is disposed so as to be supported by air.
In the present invention, by forming the buffer layer composed of an appropriate material, it is possible to orientationally grow or epitaxially grow the metallic thin film and the ferroelectric thin film on the Si substrate, and thus a small ferroelectric thin-film element with high performance can be easily formed. By etching portions of the Si substrate and the buffer layer using fine patterning techniques, the thin-film laminate composed of the metallic thin film, the ferroelectric thin film, etc., can be supported by air, and thus it is possible to easily machine a sensing part of the ferroelectric thin-film element so as to have a diaphragm structure, a hollow structure or a cantilever structure.
Preferably, the buffer layer is composed of at least one layer selected from the group consisting of an MgO layer, a Ti1-xAlxN layer (where the subscript x is 0 to about 0.4) and a YSZ layer. By epitaxially growing these materials on the Si substrate, it becomes easy to epitaxially grow the lower electrode thereon.
Preferably, the lower electrode is composed of one material selected from the group consisting of Ir, Rh, Pt, SrRuO3 and (La, Sr)CoO3. By epitaxially growing these materials on the buffer layer, it becomes possible to orientationally grow or epitaxially grow the ferroelectric thin film thereon.
Preferably, the ferroelectric thin film is composed of a perovskite compound represented by the general formula ABO3 (where A is at least one of Ba, Pb and La, and B is at least one of Ti, Zr, Mg, Nb and Zn). By orientationally growing or epitaxially growing these ferroelectrics on the substrate, a large polarization can be obtained and thus superior dielectric characteristics of the ferroelectric thin-film element can be obtained.
The ferroelectric thin-film elements can be used for various types of sensors, such as acceleration sensors and pyroelectric infrared sensors, and it will be possible to obtain smaller new sensors with higher reliability.
For the purpose of illustrating the invention, there is shown in the drawings several forms which are presently preferred, it being understood, however, that the invention is not limited to the precise arrangements and instrumentalities shown.